85.Flammability diagram

The line AB represents the concentration of a mixture that includes air and hydrocarbon gases only. All the points to the left of the line AB includes the inert gas in the mixture. Those to right of the line AB and on the line AB doesn't include inert gas.

When inert gas is added to the tank atmosphere, the concentration inside the tank changes drastically. So does the upper and lower flammable limit of the mixture. It can be seen from the diagram that the UFL and LFL narrows down to converge at point E when inert gas is added.
It can be inferred that the flammability range of a mixture decreases with addition of inert gas.

Now let's assume a mixture of HC gas, air, inert gas such that it is at point a F in the graph.

If this mixture is diluted with air (Oxygen concentration 21%), then the new concentration of the mixture will follow a straight line drawn from F to the 21% mark on oxygen percentage (X- axis). i.e. Line FA.

It can be seen from the diagram that on diluting F with air, it passes through the flammable range and hence it poses a fire hazard.

To tackle this, we need to first Purge the tank with inert gas. Since inert gas has O2 concentration of around 2%, the concentration of mixture during purging process moves along the line FJ. J is the point of  2% oxygen concentration in inert gas.

The purging is continued till the mixture reaches point H. From H if the mixture is diluted with air of 21% oxygen, it will follow the line HA, which doesn't fall in flammable range. Hence safe condition is maintained throughout the process.

While looking at diagram we come across the term critical dilution limit (Line GA). Let's try to understand this term. The line GA passes by just touching the flammable range portion of diagram. If the mixture F would have been purged any less than point K (lying between F and L), then while diluting the mixture it would pass through the flammable range.

Thus critical dilution limit is the minimum amount of purging required to ensure that the mixture doesn't fall in flammable range while diluting wit hair.

From above, It is well understood that the use of inert gas itself doesn't guarantee the safe environment of tank during tank operations, until inert gas present is above the critical limit of the environment.

To be on the safer side, it is a common practice to continue purging the mixture F even beyond the critical dilution limit, to prevent any fire hazard due to the purged mixture passing trough flammable range during dilution with air.